1. Field of the Invention
The present invention is directed toward methods for separating hydrocarbon gas constituents to more efficiently and economically separate and recover both the light, gaseous hydrocarbons and the heavier hydrocarbon liquids. The present invention provides methods for achieving essentially complete separation and recovery of propane and heavier hydrocarbon liquids. More particularly, the methods of the present invention more efficiently and more economically separate propane, propylene and heavier hydrocarbon liquids (and, if desired, ethane and ethylene) from any hydrocarbon gas stream, i.e., from natural gas or from gases from refinery or petroleum plants.
2. Description of the Background
In addition to methane, natural gas includes some heavier hydrocarbons and other impurities, e.g., carbon dioxide, nitrogen, helium, water and non-hydrocarbon acid gases. After compression and separation of these impurities, natural gas is further processed to separate and recover natural gas liquids (NGL). In fact, natural gas may include up to about fifty percent (50%) by volume of heavier hydrocarbons recovered as NGL. These heavier hydrocarbons must be separated from the methane to provide pipeline quality methane and recovered natural gas liquids. These valuable natural gas liquids comprise ethane, propane, butane and other heavier hydrocarbons. In addition to these NGL components, other gases, including hydrogen, ethylene and propylene, may be contained in gas streams from refinery or petrochemical plants.
Processes for separating hydrocarbon gas components are well known in the art. C. Collins, R. J. J. Chen and D. G. Elliot have provided an excellent, general review of NGL recovery methods in a paper presented at GasTech LNG/LPG Conference 84. This paper, entitled Trends in NGL Recovery for Natural and Associated Gases, was published by GasTech, Ltd. of Rickmansworth, England, in the transactions of the conference at pages 287-303. The pre-purified natural gas is treated by well known methods including absorption, refrigerated absorption, adsorption and condensation at cryogenic temperatures down to about -175.degree. F. Separation of the lower hydrocarbons is achieved in one or more distillation towers. The columns are often referred to as de-methanizer or de-ethanizer columns. Processes employing a de-methanizer column separate methane and other more volatile components from ethane and less volatile components in the purified gas stream. The methane fraction is recovered as a purified gas for pipeline delivery. The ethane and less volatile components, including propane, are recovered as natural gas liquids. In some applications, however, it is desirable to minimize the ethane content of the NGL. In those applications, ethane and more volatile components are separated from propane and less volatile components in a column generally known as a de-ethanizer column.
An NGL recovery plant design is highly dependent on the operating pressure of the distillation column(s). At medium to low pressures, i.e., 400 psia or lower, the recompression horsepower requirement will be so high that the process becomes uneconomical. However, at higher pressures the recovery level of hydrocarbon liquids will be significantly reduced due to the less favorable separation conditions, i.e., lower relative volatility inside the distillation column(s). Prior art methods have concentrated on operating the distillation column(s) at higher pressures, i.e., 400 psia or higher while attempting to maintain high recovery of liquid hydrocarbons. In order to achieve these goals, some systems have included two towers, one operated at higher pressure and one at lower pressure.
Many patents have been directed to methods for improving this separation technology. For example, see U.S. Pat. No. 4,596,588 describing methods for separating hydrocarbon gases using a two-column system. Many of the methods disclosed in these patents sought to improve the separation technique by either increasing the reflux flow or providing a leaner or colder reflux stream to the distillation column near the top. For example, see U.S. Pat. Nos. 4,171,964 and 4,278,457. These patents disclose that the separation process may be improved by generating more reflux at colder temperature from a portion of the feed gas by heat exchange with the overhead vapor stream from the de-methanizer column. U.S. Pat. No. 4,687,499 discloses that the warmed and compressed overhead vapor stream should be further chilled and expanded before return to the de-methanizer column as reflux. In a still further variation, U.S. Pat. No. 4,851,020 discloses a cold recycle process wherein a recycle stream containing liquid at elevated pressure is returned to the top of a de-methanizer column to improve the ethane recovery in the NGL product. All of these prior art methods attempt to improve the NGL recovery processes by either generating leaner reflux or recycling a portion of the overhead vapor from the de-methanizer column after it has been compressed to an elevated pressure.
A significant cost in NGL recovery processes is related to the refrigeration required to chill the inlet gas. Refrigeration for these low temperature recovery processes is commonly provided by external refrigeration systems using ethane or propane as refrigerants. In some applications, mixed refrigerants and cascade refrigeration cycles have been used. Refrigeration has also been provided by turbo expansion or work expansion of the compressed natural gas feed with appropriate heat exchange.
Traditionally, the gas stream is partially condensed at medium to high pressures with the help of either external propane refrigeration, a turboexpander or both. The condensed streams are further processed in a distillation column, e.g., a de-methanizer or de-ethanizer, operated at medium to low pressures to separate the lighter components from the recovered hydrocarbon liquids. Turboexpander technology has been widely used in the last 30 years to achieve higher ethane and propane recoveries in the NGL for leaner gas. For richer gas containing significant quantities of heavy hydrocarbons, a combined process of turboexpander and external propane refrigeration is the most efficient approach.
While prior art methods have been capable of recovering more than 98% of the propane, propylene and higher hydrocarbons during the ethane recovery mode, most of those methods fail to maintain the same propane recovery level when ethane is unwanted and when operated in the ethane rejection mode. Traditionally, there have been four ways to increase propane recovery while operating in the ethane rejection mode. The operating pressure of the de-ethanizer may be reduced. This approach often includes a two-stage expander design to accommodate the higher expansion ratio more efficiently. Despite requiring a significant increase in recompression horsepower, these methods are capable of recovering up to about 90 percent of the propane in the gas feed.
An alternative approach is disclosed in U.S. Pat. No. 4,251,249. The '249 patent discloses the addition of a separator at the expander discharge to partially remove methane in the gas phase so that only the liquid is sent to the de-ethanizer for further processing. Addition of an overhead condenser to the de-ethanizer minimizes the propane loss in the overhead vaporstream. However, the propane loss in the separator vapor is still too great to permit this method to achieve more than 90 percent propane recovery.
The use of a propane-free or low propane reflux in an attempt to overcome the deficiencies of the '249 patent is disclosed in U.S. Pat. Nos. 4,657,571 and 4,690,702. An improved expander discharge separator design includes the addition of a packing section and use of a cold recycle stream from the de-ethanizer overhead as reflux. This reflux improves propane recovery from the expander discharge vapor in the new packing section. The content of propane in the overhead vapor stream exiting the de-ethanizer can be minimized and controlled by the reflux flow. While recovery of more than 98 percent of the propane is achievable with this system, the recycle of methane and ethane increases both the condenser and reboiler duties. Further, the size of the de-ethanizer must be increased.
In a related approach, U.S. Pat. No. 5,568,737 suggests a system for increasing ethane recovery by recycling the residue gas stream from the residue gas compressor discharge Because the residue gas contains the least amount of propane, recycle of a significant amount of the residue gas at a much higher pressure can generate more and leaner reflux, which may permit recovery of more than 98 percent of the propane during ethane rejection operation. However, the system disclosed in the '737 patent requires a significant increase in capital and incurs much higher operating costs caused by the penalty on compression horsepower.
In yet another prior approach, a second de-ethanizer column has been added to a system designed to recover ethane. The second de-ethanizer column is added to separate out the ethane stream from the ethane plus NGL stream recovered from the upstream de-methanizer bottom. Liquid product purity is controlled by a de-ethanizer bottom reboiler and propane loss in the ethane stream is minimized by controlling the tower reflux rate. The ethane stream is combined with the de-methanizer overhead as the plant residue gas. The level of propane recovery is tied to the level of ethane recovery in the de-methanizer. In general, about 96 percent of the propane can be recovered when operating at 70-75 percent ethane recovery in the de-methanizer. Because the refrigeration used to maintain high ethane recovery is non-recoverable, both the condenser and reboiler duties are increased, along with the size of the de-ethanizer as discussed above. For purposes of comparison with the present invention, this process will be used in later discussions herein.
As can be seen from the foregoing description, the prior art has long sought methods for improving the efficiency and economy of processes for separating and recovering propane and heavier natural gas liquids from natural gas. Accordingly, there has been a long felt but unfulfilled need for more efficient, more economical methods for performing this separation. The present invention provides significant improvements in efficiency and economy, thus solving those needs.